The 1st ballade is an early work - sketches started in 1831 - Chopin was 20. Completed in 1835.
The 4th ballade is only from 7 years later but is considered a more mature work. Composed in 1842.
The 1st ballade is full of fresh ideas, vigor and beauty. A clear example of Chopin's genius.
The 4th ballade is all this and an even more sophisticated rendering of how beautiful the piano can sound, wonderful contrapuntal writing, unexpected shapes and contours, a grander scheme and deeper pathos.
To say one is better than the other is rather ridiculous, but to say that you like one better than the other and "this is why - - " works:
Sorry, I tend to be a little dogmatic about my opinions when it comes to my favorite piano pieces, and I like making controversial statements

I'll admit I actually haven't played either (I want to learn the first ballade later this year), and I probably need to listen to the fourth ballade a little more to compare the two (I've listened to the first ballade so much I practically know it by heart). Also, I'm by no means a pro pianist so maybe I'm just not mature enough to appreciate it yet.
I tend to gravitate towards songs with a more pronounced melodic line, and the Gm ballade hits the spot at that better than Fm, in my opinion. The Gm melody is full of power and beauty, and it deeply resonates with me. On a surface listening, Fm sounds more jumbled and has a less clear direction; I'm sure it has structure but it's hard to hear the structure on a first listen-through. Gm constantly returns to the same theme, developing it, and has a clear story arc that culminates in the awesome coda (it's ab-ab-ab structure reminds me of La Campanella).
I'm sure my opinions on these pieces may change as the years go by, especially after I learn them. All of Chopin's ballades are incredible works of music.